Inventory holder load detection and/or stabilization

ABSTRACT

An inventory system includes an inventory holder and a mobile drive unit. The mobile drive unit includes sensors for detecting one or more load characteristics of the inventory holder and/or includes magnets and/or magnetic surfaces for securing the inventory holder to the mobile drive unit during lifting and/or moving.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of co-pending U.S. patent applicationSer. No. 14/491,752, filed Sep. 19, 2014, entitled “INVENTORY HOLDERLOAD DETECTION AND/OR STABILIZATION”, which is incorporated herein byreference in its entirety.

BACKGROUND

Modern inventory systems, such as those in mail order warehouses, supplychain distribution centers, airport luggage systems, and custom-ordermanufacturing facilities, face significant challenges in responding torequests for inventory items. As inventory systems grow, the challengesof simultaneously completing a large number of packing, storing, andother inventory-related tasks become non-trivial. In inventory systemstasked with responding to large numbers of diverse inventory requests,inefficient utilization of system resources, including space, equipment,and manpower, can result in lower throughput, unacceptably long responsetimes, an ever-increasing backlog of unfinished tasks, and, in general,poor system performance. For example, unstable loading when movinginventory items to meet demand may cause inventory items to be dropped,resulting in damage to items, unsafe obstructions to human or robotoperators, and/or delays in processes for the dropped items to becleared. Resulting costs of such dropped items may be prohibitivelyexpensive, limiting the ability of the inventory system to operateefficiently.

BRIEF DESCRIPTION OF THE DRAWINGS

Various embodiments in accordance with the present disclosure will bedescribed with reference to the drawings, in which like referencenumerals may represent like parts, and in which:

FIG. 1 illustrates an example of a mobile drive unit magneticallycoupled with an inventory holder within an inventory system according toa particular embodiment;

FIG. 2 illustrates components of an inventory system according to aparticular embodiment;

FIG. 3 illustrates in greater detail the components of an examplemanagement module that may be utilized in particular embodiments of theinventory system shown in FIG. 2;

FIGS. 4 and 5 illustrate in greater detail an example mobile drive unitthat may be utilized in particular embodiments of the inventory systemshown in FIG. 2;

FIG. 6 illustrates in greater detail an example inventory holder thatmay be utilized in particular embodiments of the inventory system shownin FIG. 2;

FIG. 7-12 illustrate operation of various components of the mobile driveunit and the inventory holder during docking, movement and undockingaccording to a particular embodiment;

FIG. 13 illustrates an exploded perspective view of an examplearrangement of load units of a drive unit and corresponding features ofan inventory holder for stabilization or detection of load distributionaccording to a particular embodiment;

FIG. 14 illustrates components of a resource scheduling module accordingto a particular embodiment;

FIG. 15 is a flowchart illustrating an example process that can beperformed by the inventory system according to a particular embodiment;and

FIG. 16 illustrates aspects of an example environment for implementingaspects in accordance with various embodiments.

DETAILED DESCRIPTION

In the following description, various embodiments will be described. Forpurposes of explanation, specific configurations and details are setforth in order to provide a thorough understanding of the embodiments.However, it will also be apparent to one skilled in the art that theembodiments may be practiced without the specific details. Furthermore,well-known features may be omitted or simplified in order not to obscurethe embodiment being described.

Embodiments herein are directed to an inventory system having multipleinventory holders and drive units for moving the inventory holders.Specifically, features herein are directed to detecting loadcharacteristics for inventory holders and/or techniques for mitigatingnegative effects which may arise from uneven or unbalanced loaddistribution in inventory holders. Load characteristics can include loaddistribution, weight distribution, and/or a location of a center of massof the inventory holder.

In accordance with an embodiment, magnets may be utilized between amobile drive unit and an inventory holder to improve a strength ofconnection therebetween and/or to detect a load distribution on theinventory holder, such as for triggering other actions. As an exampleembodiment, FIG. 1 illustrates an example of a mobile drive unit 20magnetically coupled with an inventory holder 30. The docking head 110of the mobile drive unit 20 includes head magnets 34 that align withmagnetic surfaces 36 of the docking surface 350 of the inventory holder30 when the mobile drive unit 20 is docked with the inventory holder 30.The head magnets 34 magnetically couple with the magnetic surfaces 36 tofirmly secure the inventory holder 30 relative to the mobile drive unit20. In this way, magnetically coupling the mobile drive unit 20 with theinventory holder 30 can reduce a risk of the inventory holder 30 tippingover or otherwise negatively shifting alignment while being lifted,carried, and/or moved by the mobile drive unit 20.

Additionally, and/or alternatively, a difference in strengths ofmagnetic fields acting on the head magnets 34 from magnetic surfaces 036can indicate a different distribution of weight from the inventoryholder 30 on the mobile drive unit 20 at the respective positions of thehead magnets 34. Such a difference in weight distribution may be used toprovide an indication of the stability of the inventory holder 30 whilebeing lifted, carried, and/or moved by the mobile drive unit 20. Forexample, a difference in weight distribution may be used to determine aposition of a center of mass 32 of the inventory holder 30, which mayvary according to the distribution and respective weights of inventoryitems 40 stored in the inventory holder 30. The weight distributionand/or position of the center of mass 32 may indicate whether theinventory holder 30 is out of balance, such as if a center of mass 32 isnot located approximately at the geometrical center of the inventoryholder 34. In such unbalanced situations, the magnetic coupling betweenthe head magnets 34 and the magnetic surfaces 036 may alleviate orcounteract the imbalance of the inventory holder 30.

Additionally, or alternatively, in response to a determination that theinventory holder 30 is unbalanced, the mobile drive unit 20 may performother actions. For example, the mobile drive unit 20 may drive over anobstacle 38 in order to impart an impact to inventory items 40 stored inthe inventory holder 30 so as to shift a position of the inventory items40 within the inventory holder 30. Such a shift may adjust the positionof the center of mass 32 and improve the balance of the inventory holder30 relative to the mobile drive unit 20. As another example, the mobiledrive unit 20 may undock and redock with the inventory holder 30 tocompensate for the imbalance, such as by docking more directly beneaththe center of mass 32.

In some aspects, magnetic features such as the head magnets 34 and themagnetic surfaces 36 are only used for one of the securing or loaddetection functions. In some embodiments, other components are utilizedin addition to or in lieu of head magnets 34 and/or magnetic surfaces 36to determine a load distribution and/or a position of a center of mass32 of the inventory holder 30. Some such components are discussed ingreater detail with respect to FIGS. 13-15 below, following thedescription of FIGS. 2-12.

FIG. 2 illustrates the contents of an inventory system 10. Inventorysystem 10 includes a management module 15, one or more mobile driveunits 20, one or more inventory holders 30, and one or more inventorystations 50. Mobile drive units 20 transport inventory holders 30between points within a workspace 70 in response to commandscommunicated by management module 15. Each inventory holder 30 storesone or more types of inventory items. As a result, inventory system 10is capable of moving inventory items between locations within workspace70 to facilitate the entry, processing, and/or removal of inventoryitems from inventory system 10 and the completion of other tasksinvolving inventory items.

Management module 15 assigns tasks to appropriate components ofinventory system 10 and coordinates operation of the various componentsin completing the tasks. These tasks may relate not only to the movementand processing of inventory items, but also to the management andmaintenance of the components of inventory system 10. For example,management module 15 may assign portions of workspace 70 as parkingspaces for mobile drive units 20, the scheduled recharge or replacementof mobile drive unit batteries, the storage of empty inventory holders30, or any other operations associated with the functionality supportedby inventory system 10 and its various components. Management module 15may select components of inventory system 10 to perform these tasks andcommunicate appropriate commands and/or data to the selected componentsto facilitate completion of these operations. Although shown in FIG. 2as a single, discrete component, management module 15 may representmultiple components and may represent or include portions of mobiledrive units 20 or other elements of inventory system 10. As a result,any or all of the interaction between a particular mobile drive unit 20and management module 15 that is described below may, in particularembodiments, represent peer-to-peer communication between that mobiledrive unit 20 and one or more other mobile drive units 20. The contentsand operation of an example embodiment of management module 15 arediscussed further below with respect to FIG. 3.

Mobile drive units 20 move inventory holders 30 between locations withinworkspace 70. Mobile drive units 20 may represent any devices orcomponents appropriate for use in inventory system 10 based on thecharacteristics and configuration of inventory holders 30 and/or otherelements of inventory system 10. In a particular embodiment of inventorysystem 10, mobile drive units 20 represent independent, self-powereddevices configured to freely move about workspace 70. Examples of suchinventory systems are disclosed in U.S. Patent Publication No.2012/0143427, published on Jun. 7, 2012, titled “SYSTEM AND METHOD FORPOSITIONING A MOBILE DRIVE UNIT” and U.S. Pat. No. 8,280,547, issued onOct. 2, 2012, titled “METHOD AND SYSTEM FOR TRANSPORTING INVENTORYITEMS”, the entire disclosures of which are herein incorporated byreference. In alternative embodiments, mobile drive units 20 representelements of a tracked inventory system configured to move inventoryholder 30 along tracks, rails, cables, crane system, or other guidanceor support elements traversing workspace 70. In such an embodiment,mobile drive units 20 may receive power and/or support through aconnection to the guidance elements, such as a powered rail.Additionally, in particular embodiments of inventory system 10 mobiledrive units 20 may be configured to utilize alternative conveyanceequipment to move within workspace 70 and/or between separate portionsof workspace 70. The contents and operation of an example embodiment ofa mobile drive unit 20 are discussed further below with respect to FIGS.4 and 5.

Additionally, mobile drive units 20 may be capable of communicating withmanagement module 15 to receive information identifying selectedinventory holders 30, transmit the locations of mobile drive units 20,or exchange any other suitable information to be used by managementmodule 15 or mobile drive units 20 during operation. Mobile drive units20 may communicate with management module 15 wirelessly, using wiredconnections between mobile drive units 20 and management module 15,and/or in any other appropriate manner. As one example, particularembodiments of mobile drive unit 20 may communicate with managementmodule 15 and/or with one another using 802.11, Bluetooth, or InfraredData Association (IrDA) standards, or any other appropriate wirelesscommunication protocol. As another example, in a tracked inventorysystem 10, tracks or other guidance elements upon which mobile driveunits 20 move may be wired to facilitate communication between mobiledrive units 20 and other components of inventory system 10. Furthermore,as noted above, management module 15 may include components ofindividual mobile drive units 20. Thus, for the purposes of thisdescription and the claims that follow, communication between managementmodule 15 and a particular mobile drive unit 20 may representcommunication between components of a particular mobile drive unit 20.In general, mobile drive units 20 may be powered, propelled, andcontrolled in any manner appropriate based on the configuration andcharacteristics of inventory system 10.

Inventory holders 30 store inventory items. In a particular embodiment,inventory holders 30 include multiple storage bins with each storage bincapable of holding one or more types of inventory items. Inventoryholders 30 are capable of being carried, rolled, and/or otherwise movedby mobile drive units 20. In particular embodiments, inventory holder 30may provide additional propulsion to supplement that provided by mobiledrive unit 20 when moving inventory holder 30.

Additionally, in particular embodiments, inventory items 40 may alsohang from hooks or bars (not shown) within or on inventory holder 30. Ingeneral, inventory holder 30 may store inventory items 40 in anyappropriate manner within inventory holder 30 and/or on the externalsurface of inventory holder 30.

Additionally, each inventory holder 30 may include a plurality of faces,and each bin may be accessible through one or more faces of theinventory holder 30. For example, in a particular embodiment, inventoryholder 30 includes four faces. In such an embodiment, bins located at acorner of two faces may be accessible through either of those two faces,while each of the other bins is accessible through an opening in one ofthe four faces. Mobile drive unit 20 may be configured to rotateinventory holder 30 at appropriate times to present a particular faceand the bins associated with that face to an operator or othercomponents of inventory system 10.

Inventory items represent any objects suitable for storage, retrieval,and/or processing in an automated inventory system 10. For the purposesof this description, “inventory items” may represent any one or moreobjects of a particular type that are stored in inventory system 10.Thus, a particular inventory holder 30 is currently “storing” aparticular inventory item if the inventory holder 30 currently holds oneor more units of that type. As one example, inventory system 10 mayrepresent a mail order warehouse facility, and inventory items mayrepresent merchandise stored in the warehouse facility. Duringoperation, mobile drive units 20 may retrieve inventory holders 30containing one or more inventory items requested in an order to bepacked for delivery to a customer or inventory holders 30 carryingpallets containing aggregated collections of inventory items forshipment. Moreover, in particular embodiments of inventory system 10,boxes containing completed orders may themselves represent inventoryitems.

In particular embodiments, inventory system 10 may also include one ormore inventory stations 50. Inventory stations 50 represent locationsdesignated for the completion of particular tasks involving inventoryitems. Such tasks may include the removal of inventory items frominventory holders 30, the introduction of inventory items into inventoryholders 30, the counting of inventory items in inventory holders 30, thedecomposition of inventory items (e.g., from pallet- or case-sizedgroups to individual inventory items), the consolidation of inventoryitems between inventory holders 30, and/or the processing or handling ofinventory items in any other suitable manner. In particular embodiments,inventory stations 50 may just represent the physical locations where aparticular task involving inventory items can be completed withinworkspace 70. In alternative embodiments, inventory stations 50 mayrepresent both the physical location and also any appropriate equipmentfor processing or handling inventory items, such as scanners formonitoring the flow of inventory items in and out of inventory system10, communication interfaces for communicating with management module15, and/or any other suitable components. Inventory stations 50 may becontrolled, entirely or in part, by human operators or may be fullyautomated. Moreover, the human or automated operators of inventorystations 50 may be capable of performing certain tasks to inventoryitems, such as packing, counting, or transferring inventory items, aspart of the operation of inventory system 10.

Workspace 70 represents an area associated with inventory system 10 inwhich mobile drive units 20 can move and/or inventory holders 30 can bestored. For example, workspace 70 may represent all or part of the floorof a mail-order warehouse in which inventory system 10 operates. In someembodiments, workspace 70 includes multiple floors, and some combinationof ramps, elevators, conveyors, and/or other devices are provided tofacilitate movement of mobile drive units 20 and/or other components ofthe inventory system 10 between the multiple floors. Although FIG. 2shows, for the purposes of illustration, an embodiment of inventorysystem 10 in which workspace 70 includes a fixed, predetermined, andfinite physical space, particular embodiments of inventory system 10 mayinclude mobile drive units 20 and inventory holders 30 that areconfigured to operate within a workspace 70 that is of variabledimensions and/or an arbitrary geometry. While FIG. 2 illustrates aparticular embodiment of inventory system 10 in which workspace 70 isentirely enclosed in a building, alternative embodiments may utilizeworkspaces 70 in which some or all of the workspace 70 is locatedoutdoors, within a vehicle (such as a cargo ship), or otherwiseunconstrained by any fixed structure.

In operation, management module 15 selects appropriate components tocomplete particular tasks and transmits task assignments 18 to theselected components to trigger completion of the relevant tasks. Eachtask assignment 18 defines one or more tasks to be completed by aparticular component. These tasks may relate to the retrieval, storage,replenishment, and counting of inventory items and/or the management ofmobile drive units 20, inventory holders 30, inventory stations 50 andother components of inventory system 10. Depending on the component andthe task to be completed, a particular task assignment 18 may identifylocations, components, and/or actions associated with the correspondingtask and/or any other appropriate information to be used by the relevantcomponent in completing the assigned task.

In particular embodiments, management module 15 generates taskassignments 18 based, in part, on inventory requests that managementmodule 15 receives from other components of inventory system 10 and/orfrom external components in communication with management module 15.These inventory requests identify particular operations to be completedinvolving inventory items stored or to be stored within inventory system10 and may represent communication of any suitable form. For example, inparticular embodiments, an inventory request may represent a shippingorder specifying particular inventory items that have been purchased bya customer and that are to be retrieved from inventory system 10 forshipment to the customer. Management module 15 may also generate taskassignments 18 independently of such inventory requests, as part of theoverall management and maintenance of inventory system 10. For example,management module 15 may generate task assignments 18 in response to theoccurrence of a particular event (e.g., in response to a mobile driveunit 20 requesting a space to park), according to a predeterminedschedule (e.g., as part of a daily start-up routine), or at anyappropriate time based on the configuration and characteristics ofinventory system 10. After generating one or more task assignments 18,management module 15 transmits the generated task assignments 18 toappropriate components for completion of the corresponding task. Therelevant components then execute their assigned tasks.

With respect to mobile drive units 20 specifically, management module 15may, in particular embodiments, communicate task assignments 18 toselected mobile drive units 20 that identify one or more destinationsfor the selected mobile drive units 20. Management module 15 may selecta mobile drive unit 20 to assign the relevant task based on the locationor state of the selected mobile drive unit 20, an indication that theselected mobile drive unit 20 has completed a previously-assigned task,a predetermined schedule, and/or any other suitable consideration. Thesedestinations may be associated with an inventory request the managementmodule 15 is executing or a management objective the management module15 is attempting to fulfill. For example, the task assignment may definethe location of an inventory holder 30 to be retrieved, an inventorystation 50 to be visited, a storage location where the mobile drive unit20 should park until receiving another task, or a location associatedwith any other task appropriate based on the configuration,characteristics, and/or state of inventory system 10, as a whole, orindividual components of inventory system 10. For example, in particularembodiments, such decisions may be based on the popularity of particularinventory items, the staffing of a particular inventory station 50, thetasks currently assigned to a particular mobile drive unit 20, and/orany other appropriate considerations.

As part of completing these tasks mobile drive units 20 may dock withand transport inventory holders 30 within workspace 70. Mobile driveunits 20 may dock with inventory holders 30 by connecting to, lifting,and/or otherwise interacting with inventory holders 30 in any othersuitable manner so that, when docked, mobile drive units 20 are coupledto and/or support inventory holders 30 and can move inventory holders 30within workspace 70. While the description below focuses on particularembodiments of mobile drive unit 20 and inventory holder 30 that areconfigured to dock in a particular manner, alternative embodiments ofmobile drive unit 20 and inventory holder 30 may be configured to dockin any manner suitable to allow mobile drive unit 20 to move inventoryholder 30 within workspace 70. Additionally, as noted below, inparticular embodiments, mobile drive units 20 represent all or portionsof inventory holders 30. In such embodiments, mobile drive units 20 maynot dock with inventory holders 30 before transporting inventory holders30 and/or mobile drive units 20 may each remain continually docked witha particular inventory holder 30.

While the appropriate components of inventory system 10 completeassigned tasks, management module 15 may interact with the relevantcomponents to ensure the efficient use of space, equipment, manpower,and other resources available to inventory system 10. As one specificexample of such interaction, management module 15 is responsible, inparticular embodiments, for planning the paths mobile drive units 20take when moving within workspace 70 and for allocating use of aparticular portion of workspace 70 to a particular mobile drive unit 20for purposes of completing an assigned task. In such embodiments, mobiledrive units 20 may, in response to being assigned a task, request a pathto a particular destination associated with the task. Moreover, whilethe description below focuses on one or more embodiments in which mobiledrive unit 20 requests paths from management module 15, mobile driveunit 20 may, in alternative embodiments, generate its own paths.

Components of inventory system 10 may provide information to managementmodule 15 regarding their current state, other components of inventorysystem 10 with which they are interacting, and/or other conditionsrelevant to the operation of inventory system 10. This may allowmanagement module 15 to utilize feedback from the relevant components toupdate algorithm parameters, adjust policies, or otherwise modify itsdecision-making to respond to changes in operating conditions or theoccurrence of particular events.

In addition, while management module 15 may be configured to managevarious aspects of the operation of the components of inventory system10, in particular embodiments, the components themselves may also beresponsible for decision-making relating to certain aspects of theiroperation, thereby reducing the processing load on management module 15.

Thus, based on its knowledge of the location, current state, and/orother characteristics of the various components of inventory system 10and an awareness of all the tasks currently being completed, managementmodule 15 can generate tasks, allot usage of system resources, andotherwise direct the completion of tasks by the individual components ina manner that optimizes operation from a system-wide perspective.Moreover, by relying on a combination of both centralized, system-widemanagement and localized, component-specific decision-making, particularembodiments of inventory system 10 may be able to support a number oftechniques for efficiently executing various aspects of the operation ofinventory system 10. As a result, particular embodiments of managementmodule 15 may, by implementing one or more management techniquesdescribed below, enhance the efficiency of inventory system 10 and/orprovide other operational benefits.

FIG. 3 illustrates in greater detail the components of a particularembodiment of management module 15. As shown, the example embodimentincludes a resource scheduling module 92, a route planning module 94, asegment reservation module 96, an inventory module 97, a communicationinterface module 98, a processor 90, and a memory 91. Management module15 may represent a single component, multiple components located at acentral location within inventory system 10, or multiple componentsdistributed throughout inventory system 10. For example, managementmodule 15 may represent components of one or more mobile drive units 20that are capable of communicating information between the mobile driveunits 20 and coordinating the movement of mobile drive units 20 withinworkspace 70. In general, management module 15 may include anyappropriate combination of hardware and/or software suitable to providethe described functionality.

Processor 90 is operable to execute instructions associated with thefunctionality provided by management module 15. Processor 90 maycomprise one or more general purpose computers, dedicatedmicroprocessors, or other processing devices capable of communicatingelectronic information. Examples of processor 90 include one or moreapplication-specific integrated circuits (ASICs), field programmablegate arrays (FPGAs), digital signal processors (DSPs) and any othersuitable specific or general purpose processors.

Memory 91 stores processor instructions, inventory requests, reservationinformation, state information for the various components of inventorysystem 10 and/or any other appropriate values, parameters, orinformation utilized by management module 15 during operation. Memory 91may represent any collection and arrangement of volatile or nonvolatile,local or remote devices suitable for storing data. Examples of memory 91include, but are not limited to, random access memory (RAM) devices,read only memory (ROM) devices, magnetic storage devices, opticalstorage devices, or any other suitable data storage devices.

Resource scheduling module 92 processes received inventory requests andgenerates one or more assigned tasks to be completed by the componentsof inventory system 10. Resource scheduling module 92 may also selectone or more appropriate components for completing the assigned tasksand, using communication interface module 98, communicate the assignedtasks to the relevant components. Additionally, resource schedulingmodule 92 may also be responsible for generating assigned tasksassociated with various management operations, such as prompting mobiledrive units 20 to recharge batteries or have batteries replaced,instructing inactive mobile drive units 20 to park in a location outsidethe anticipated traffic flow or a location near the anticipated site offuture tasks, and/or directing mobile drive units 20 selected for repairor maintenance to move towards a designated maintenance station.

Route planning module 94 receives route requests from mobile drive units20. These route requests identify one or more destinations associatedwith a task the requesting mobile drive unit 20 is executing. Inresponse to receiving a route request, route planning module 94generates a path to one or more destinations identified in the routerequest. Route planning module 94 may implement any appropriatealgorithms utilizing any appropriate parameters, factors, and/orconsiderations to determine the appropriate path. After generating anappropriate path, route planning module 94 transmits a route responseidentifying the generated path to the requesting mobile drive unit 20using communication interface module 98.

Segment reservation module 96 receives reservation requests from mobiledrive units 20 attempting to move along paths generated by routeplanning module 94. These reservation requests request the use of aparticular portion of workspace 70 (referred to herein as a “segment”)to allow the requesting mobile drive unit 20 to avoid collisions withother mobile drive units 20 while moving across the reserved segment. Inresponse to received reservation requests, segment reservation module 96transmits a reservation response granting or denying the reservationrequest to the requesting mobile drive unit 20 using the communicationinterface module 98.

The inventory module 97 maintains information about the location andnumber of inventory items 40 in the inventory system 10. Information canbe maintained about the number of inventory items 40 in a particularinventory holder 30, and the maintained information can include thelocation of those inventory items 40 in the inventory holder 30. Theinventory module 97 can also communicate with the mobile drive units 20,utilizing task assignments 18 to maintain, replenish, or move inventoryitems 40 within the inventory system 10.

Communication interface module 98 facilitates communication betweenmanagement module 15 and other components of inventory system 10,including reservation responses, reservation requests, route requests,route responses, and task assignments. These reservation responses,reservation requests, route requests, route responses, and taskassignments may represent communication of any form appropriate based onthe capabilities of management module 15 and may include any suitableinformation. Depending on the configuration of management module 15,communication interface module 98 may be responsible for facilitatingeither or both of wired and wireless communication between managementmodule 15 and the various components of inventory system 10. Inparticular embodiments, management module 15 may communicate usingcommunication protocols such as 802.11, Bluetooth, or Infrared DataAssociation (IrDA) standards. Furthermore, management module 15 may, inparticular embodiments, represent a portion of mobile drive unit 20 orother components of inventory system 10. In such embodiments,communication interface module 98 may facilitate communication betweenmanagement module 15 and other parts of the same system component.

In general, resource scheduling module 92, route planning module 94,segment reservation module 96, inventory module 97, and communicationinterface module 98 may each represent any appropriate hardware and/orsoftware suitable to provide the described functionality. In addition,as noted above, management module 15 may, in particular embodiments,represent multiple different discrete components and any or all ofresource scheduling module 92, route planning module 94, segmentreservation module 96, inventory module 97, and communication interfacemodule 98 may represent components physically separate from theremaining elements of management module 15. Moreover, any two or more ofresource scheduling module 92, route planning module 94, segmentreservation module 96, inventory module 97, and communication interfacemodule 98 may share common components. For example, in particularembodiments, resource scheduling module 92, route planning module 94,segment reservation module 96, and inventory module 97 representcomputer processes executing on processor 90 and communication interfacemodule 98 comprises a wireless transmitter, a wireless receiver, and arelated computer process executing on processor 90.

FIGS. 4 and 5 illustrate in greater detail the components of aparticular embodiment of mobile drive unit 20. In particular, FIGS. 4and 5 include a front and side view of an example mobile drive unit 20.Mobile drive unit 20 includes a docking head 110, a drive module 120, adocking actuator 130, and a control module 170. Additionally, mobiledrive unit 20 may include one or more sensors configured to detect ordetermine the location of mobile drive unit 20, inventory holder 30,and/or other appropriate elements of inventory system 10. In theillustrated embodiment, mobile drive unit 20 includes a position sensor140, a holder sensor 150, an obstacle sensor 160, and an identificationsignal transmitter 162.

Docking head 110, in particular embodiments of mobile drive unit 20,couples mobile drive unit 20 to inventory holder 30 and/or supportsinventory holder 30 when mobile drive unit 20 is docked to inventoryholder 30. Docking head 110 may additionally allow mobile drive unit 20to maneuver inventory holder 30, such as by lifting inventory holder 30,propelling inventory holder 30, rotating inventory holder 30, and/ormoving inventory holder 30 in any other appropriate manner. Docking head110 may also include any appropriate combination of components, such asribs, spikes, and/or corrugations, to facilitate such manipulation ofinventory holder 30. For example, in particular embodiments, dockinghead 110 may include a high-friction portion that abuts a portion ofinventory holder 30 while mobile drive unit 20 is docked to inventoryholder 30. In such embodiments, frictional forces created between thehigh-friction portion of docking head 110 and a surface of inventoryholder 30 may induce translational and rotational movement in inventoryholder 30 when docking head 110 moves and rotates, respectively. As aresult, mobile drive unit 20 may be able to manipulate inventory holder30 by moving or rotating docking head 110, either independently or as apart of the movement of mobile drive unit 20 as a whole.

Drive module 120 propels mobile drive unit 20 and, when mobile driveunit 20 and inventory holder 30 are docked, inventory holder 30. Drivemodule 120 may represent any appropriate collection of componentsoperable to propel mobile drive unit 20. For example, in the illustratedembodiment, drive module 120 includes motorized axle 122, a pair ofmotorized wheels 124, and a pair of stabilizing wheels 126. Onemotorized wheel 124 is located at each end of motorized axle 122, andone stabilizing wheel 126 is positioned at each end of mobile drive unit20.

Docking actuator 130 moves docking head 110 towards inventory holder 30to facilitate docking of mobile drive unit 20 and inventory holder 30.Docking actuator 130 may also be capable of adjusting the position ororientation of docking head 110 in other suitable manners to facilitatedocking. Docking actuator 130 may include any appropriate components,based on the configuration of mobile drive unit 20 and inventory holder30, for moving docking head 110 or otherwise adjusting the position ororientation of docking head 110. For example, in the illustratedembodiment, docking actuator 130 includes a motorized shaft (not shown)attached to the center of docking head 110. The motorized shaft isoperable to lift docking head 110 as appropriate for docking withinventory holder 30.

Drive module 120 may be configured to propel mobile drive unit 20 in anyappropriate manner. For example, in the illustrated embodiment,motorized wheels 124 are operable to rotate in a first direction topropel mobile drive unit 20 in a forward direction. Motorized wheels 124are also operable to rotate in a second direction to propel mobile driveunit 20 in a backward direction. In the illustrated embodiment, drivemodule 120 is also configured to rotate mobile drive unit 20 by rotatingmotorized wheels 124 in different directions from one another or byrotating motorized wheels 124 at different speed from one another.

Position sensor 140 represents one or more sensors, detectors, or othercomponents suitable for determining the location of mobile drive unit 20in any appropriate manner. For example, in particular embodiments, theworkspace 70 associated with inventory system 10 includes a number offiducial marks that mark points on a two-dimensional grid that coversall or a portion of workspace 70. In such embodiments, position sensor140 may include a camera and suitable image- and/or video-processingcomponents, such as an appropriately-programmed digital signalprocessor, to allow position sensor 140 to detect fiducial marks withinthe camera's field of view. Control module 170 may store locationinformation that position sensor 140 updates as position sensor 140detects fiducial marks. As a result, position sensor 140 may utilizefiducial marks to maintain an accurate indication of the location mobiledrive unit 20 and to aid in navigation when moving within workspace 70.

Holder sensor 150 represents one or more sensors, detectors, or othercomponents suitable for detecting inventory holder 30 and/ordetermining, in any appropriate manner, the location of inventory holder30, as an absolute location or as a position relative to mobile driveunit 20. Holder sensor 150 may be capable of detecting the location of aparticular portion of inventory holder 30 or inventory holder 30 as awhole. Mobile drive unit 20 may then use the detected information fordocking with or otherwise interacting with inventory holder 30.

Obstacle sensor 160 represents one or more sensors capable of detectingobjects located in one or more different directions in which mobiledrive unit 20 is capable of moving. Obstacle sensor 160 may utilize anyappropriate components and techniques, including optical, radar, sonar,pressure-sensing and/or other types of detection devices appropriate todetect objects located in the direction of travel of mobile drive unit20. In particular embodiments, obstacle sensor 160 may transmitinformation describing objects it detects to control module 170 to beused by control module 170 to identify obstacles and to take appropriateremedial actions to prevent mobile drive unit 20 from colliding withobstacles and/or other objects.

Obstacle sensor 160 may also detect signals transmitted by other mobiledrive units 20 operating in the vicinity of the illustrated mobile driveunit 20. For example, in particular embodiments of inventory system 10,one or more mobile drive units 20 may include an identification signaltransmitter 162 that transmits a drive identification signal. The driveidentification signal indicates to other mobile drive units 20 that theobject transmitting the drive identification signal is in fact a mobiledrive unit. Identification signal transmitter 162 may be capable oftransmitting infrared, ultraviolet, audio, visible light, radio, and/orother suitable signals that indicate to recipients that the transmittingdevice is a mobile drive unit 20.

Additionally, in particular embodiments, obstacle sensor 160 may also becapable of detecting state information transmitted by other mobile driveunits 20. For example, in particular embodiments, identification signaltransmitter 162 may be capable of including state information relatingto mobile drive unit 20 in the transmitted identification signal. Thisstate information may include, but is not limited to, the position,velocity, direction, and the braking capabilities of the transmittingmobile drive unit 20. In particular embodiments, mobile drive unit 20may use the state information transmitted by other mobile drive units toavoid collisions when operating in close proximity with those othermobile drive units.

Control module 170 monitors and/or controls operation of drive module120 and docking actuator 130. Control module 170 may also receiveinformation from sensors such as position sensor 140 and holder sensor150 and adjust the operation of drive module 120, docking actuator 130,and/or other components of mobile drive unit 20 based on thisinformation. Additionally, in particular embodiments, mobile drive unit20 may be configured to communicate with a management device ofinventory system 10 and control module 170 may receive commandstransmitted to mobile drive unit 20 and communicate information back tothe management device utilizing appropriate communication components ofmobile drive unit 20. Control module 170 may include any appropriatehardware and/or software suitable to provide the describedfunctionality. In particular embodiments, control module 170 includes ageneral-purpose microprocessor programmed to provide the describedfunctionality. Additionally, control module 170 may include all orportions of docking actuator 130, drive module 120, position sensor 140,and/or holder sensor 150, and/or share components with any of theseelements of mobile drive unit 20.

Moreover, in particular embodiments, control module 170 may includehardware and software located in components that are physically distinctfrom the device that houses drive module 120, docking actuator 130,and/or the other components of mobile drive unit 20 described above. Forexample, in particular embodiments, each mobile drive unit 20 operatingin inventory system 10 may be associated with a software process(referred to here as a “drive agent”) operating on a server that is incommunication with the device that houses drive module 120, dockingactuator 130, and other appropriate components of mobile drive unit 20.This drive agent may be responsible for requesting and receiving tasks,requesting and receiving routes, transmitting state informationassociated with mobile drive unit 20, and/or otherwise interacting withmanagement module 15 and other components of inventory system 10 onbehalf of the device that physically houses drive module 120, dockingactuator 130, and the other appropriate components of mobile drive unit20. As a result, for the purposes of this description and the claimsthat follow, the term “mobile drive unit” includes software and/orhardware, such as agent processes, that provides the describedfunctionality on behalf of mobile drive unit 20 but that may be locatedin physically distinct devices from the drive module 120, dockingactuator 130, and/or the other components of mobile drive unit 20described above.

While FIGS. 4 and 5 illustrate a particular embodiment of mobile driveunit 20 containing certain components and configured to operate in aparticular manner, mobile drive unit 20 may represent any appropriatecomponent and/or collection of components configured to transport and/orfacilitate the transport of inventory holders 30. As another example,mobile drive unit 20 may represent part of an overhead crane system inwhich one or more crane assemblies are capable of moving within anetwork of wires or rails to a position suitable to dock with aparticular inventory holder 30. After docking with inventory holder 30,the crane assembly may then lift inventory holder 30 and move inventoryto another location for purposes of completing an assigned task.

Furthermore, in particular embodiments, mobile drive unit 20 mayrepresent all or a portion of inventory holder 30. Inventory holder 30may include motorized wheels or any other components suitable to allowinventory holder 30 to propel itself. As one specific example, a portionof inventory holder 30 may be responsive to magnetic fields. Inventorysystem 10 may be able to generate one or more controlled magnetic fieldscapable of propelling, maneuvering, and/or otherwise controlling theposition of inventory holder 30 as a result of the responsive portion ofinventory holder 30. In such embodiments, mobile drive unit 20 mayrepresent the responsive portion of inventory holder 30 and/or thecomponents of inventory system 10 responsible for generating andcontrolling these magnetic fields. While this description providesseveral specific examples, mobile drive unit 20 may, in general,represent any appropriate component and/or collection of componentsconfigured to transport and/or facilitate the transport of inventoryholders 30.

FIG. 6 illustrates in greater detail the components of a particularembodiment of inventory holder 30. In particular, FIG. 6 illustrates thestructure and contents of one side of an example inventory holder 30. Ina particular embodiment, inventory holder 30 may comprise any number offaces with similar or different structure. As illustrated, inventoryholder 30 includes a frame 310, a plurality of legs 328, and dockingsurface 350.

Frame 310 holds inventory items 40. Frame 310 provides storage space forstoring inventory items 40 external or internal to frame 310. Thestorage space provided by frame 310 may be divided into a plurality ofinventory bins 320, each capable of holding inventory items 40.Inventory bins 320 may include any appropriate storage elements, such asbins, compartments, or hooks.

In a particular embodiment, frame 310 is composed of a plurality oftrays 322 stacked upon one another and attached to or stacked on a base318. In such an embodiment, inventory bins 320 may be formed by aplurality of adjustable dividers 324 that may be moved to resize one ormore inventory bins 320. In alternative embodiments, frame 310 mayrepresent a single inventory bin 320 that includes a single tray 322 andno adjustable dividers 324. In some embodiments, a frame 310 and/or aninventory bin 320 of an inventory holder 30 may be designed toaccommodate one or more pallets for inventory items 40. Additionally, inparticular embodiments, frame 310 may represent a load-bearing surfacemounted on mobility element 330. Inventory items 40 may be stored onsuch an inventory holder 30 by being placed on frame 310. In general,frame 310 may include storage internal and/or external storage spacedivided into any appropriate number of inventory bins 320 in anyappropriate manner.

Additionally, in a particular embodiment, frame 310 may include aplurality of device openings 326 that allow mobile drive unit 20 toposition docking head 110 adjacent docking surface 350. The size, shape,and placement of device openings 326 may be determined based on thesize, the shape, and other characteristics of the particular embodimentof mobile drive unit 20 and/or inventory holder 30 utilized by inventorysystem 10. For example, in the illustrated embodiment, frame 310includes four legs 328 that form device openings 326 and allow mobiledrive unit 20 to position mobile drive unit 20 under frame 310 andadjacent to docking surface 350. The length of legs 328 may bedetermined based on a height of mobile drive unit 20.

Docking surface 350 comprises a portion of inventory holder 30 thatcouples to, abuts, and/or rests upon a portion of docking head 110, whenmobile drive unit 20 is docked to inventory holder 30. Additionally,docking surface 350 supports a portion or all of the weight of inventoryholder 30 while inventory holder 30 is docked with mobile drive unit 20.The composition, shape, and/or texture of docking surface 350 may bedesigned to facilitate maneuvering of inventory holder 30 by mobiledrive unit 20. For example, as noted above, in particular embodiments,docking surface 350 may comprise a high-friction portion. When mobiledrive unit 20 and inventory holder 30 are docked, frictional forcesinduced between docking head 110 and this high-friction portion mayallow mobile drive unit 20 to maneuver inventory holder 30.Additionally, in particular embodiments, docking surface 350 may includeappropriate components suitable to receive a portion of docking head110, couple inventory holder 30 to mobile drive unit 20, and/orfacilitate control of inventory holder 30 by mobile drive unit 20.

Holder identifier 360 marks a predetermined portion of inventory holder30 and mobile drive unit 20 may use holder identifier 360 to align withinventory holder 30 during docking and/or to determine the location ofinventory holder 30. More specifically, in particular embodiments,mobile drive unit 20 may be equipped with components, such as holdersensor 150, that can detect holder identifier 360 and determine itslocation relative to mobile drive unit 20. As a result, mobile driveunit 20 may be able to determine the location of inventory holder 30 asa whole. For example, in particular embodiments, holder identifier 360may represent a reflective marker that is positioned at a predeterminedlocation on inventory holder 30 and that holder sensor 150 can opticallydetect using an appropriately-configured camera.

Depending on the configuration and characteristics of mobile drive unit20 and inventory system 10, mobile drive unit 20 may move inventoryholder 30 using a variety of appropriate methods. In a particularembodiment, mobile drive unit 20 is capable of moving inventory holder30 along a two-dimensional grid, combining movement along straight-linesegments with ninety-degree rotations and arcing paths to transportinventory holder 30 from the first location to the second location.Additionally, while moving, mobile drive unit 20 may use fixed objectslocated in the workspace as reference points to assist in navigation.For example, in particular embodiments, inventory system 10 includesmultiple fiducial marks. Mobile drive unit 20 may be configured todetect fiducial marks and to determine the location of mobile drive unit20 and/or measure its movement based on the detection of fiducial marks.

After mobile drive unit 20 arrives at the second location, mobile driveunit 20 may perform appropriate operations to facilitate access toinventory items 40 stored in inventory holder 30. For example, mobiledrive unit 20 may rotate inventory holder 30 to present a particularface of inventory holder 30 to an operator of inventory system 10 orother suitable party, such as a packer selecting inventory items 40 frominventory holder 30. Mobile drive unit 20 may also undock from inventoryholder 30. Alternatively, instead of undocking at the second location,mobile drive unit 20 may transport inventory holder 30 back to the firstlocation or to a third location after any appropriate actions have beentaken involving inventory items 40. For example, after a packer hasremoved particular inventory items 40 from inventory holder 30, mobiledrive unit 20 may return inventory holder 30 to its original storagelocation, a new storage location, or another inventory station. Mobiledrive unit 20 may then undock from inventory holder 30 at this newlocation.

FIGS. 7-12 illustrate operation of particular embodiments of mobiledrive unit 20 and inventory holder 30 during docking, movement, andundocking.

FIG. 7 illustrates mobile drive unit 20 and inventory holder 30 prior todocking. As noted above with respect to FIG. 1, mobile drive unit 20 mayreceive a command that identifies a location for a particular inventoryholder 30. Mobile drive unit 20 may then move to the location specifiedin the command. Additionally, mobile drive unit 20 may utilize positionsensor 140 to determine the location of mobile drive unit 20 to assistin navigating to the location of inventory holder 30.

In particular, FIG. 7 shows mobile drive unit 20 and inventory holder 30as mobile drive unit 20 approaches the storage location identified bythe received command. In the illustrated embodiment, the reference pointis marked by fiducial mark 450A which comprises a surface operable toreflect light and which, as a result, can be detected by particularembodiments of position sensor 140 when mobile drive unit 20 ispositioned over or approximately over fiducial mark 450A. As notedabove, the illustrated embodiment of mobile drive unit 20 utilizesoptical sensors, including a camera and appropriate image- and/or videoprocessing components, to detect fiducial marks 450.

FIG. 8 illustrates mobile drive unit 20 and inventory holder 30 oncemobile drive unit 20 reaches fiducial mark 450A. Because, in theillustrated example, fiducial mark 450A marks the location of thereference point to which mobile drive unit 20 is destined, mobile driveunit 20 begins the docking process once mobile drive unit 20 reachesfiducial mark 450A. In the illustrated example, mobile drive unit 20 isconfigured to dock with inventory holder 30 from a position beneathinventory holder 30 and, as a result, inventory holder 30 is stored sothat docking surface 350 is located directly above fiducial mark 450A.

FIG. 9 illustrates operation of mobile drive unit 20 in docking withinventory holder 30. After positioning itself over fiducial mark 450A,mobile drive unit 20 begins the docking process. In the illustratedexample, the docking process includes mobile drive unit 20 raisingdocking head 110 towards docking surface 350, as indicated by arrow 410.Additionally, in the illustrated example, mobile drive unit 20 andinventory holder 30 are configured so that mobile drive unit 20 liftsinventory holder 30 off the ground when mobile drive unit 20 docks withinventory holder 30 and, as a result, mobile drive unit 20 supports theweight of inventory holder 30 while mobile drive unit 20 is docked toinventory holder 30.

FIG. 10 illustrates operation of mobile drive unit 20 after docking withinventory holder 30. Mobile drive unit 20 is capable of inducingtranslational and/or rotational movement in inventory holder 30 whilemobile drive unit 20 is docked with inventory holder 30. For example, inthe illustrated embodiment, inventory holder 30 is supported by mobiledrive unit 20 while the two components are docked and mobile drive unit20 is capable of inducing translational and/or rotational movement ininventory holder 30 by moving or rotating itself or some sub-componentof itself, such as docking head 110. As a result, while mobile driveunit 20 and inventory holder 30 are docked mobile drive unit 20 may moveinventory holder 30 to a requested destination based on commandsreceived by mobile drive unit 20, as suggested by arrow 420.

Once mobile drive unit 20 and inventory holder 30 arrive at thedestination, mobile drive unit 20 may additionally rotate inventoryholder 30 to present a particular face of inventory holder 30 to apacker or otherwise maneuver inventory holder 30 to allow access toinventory items 40 stored by inventory holder 30. Mobile drive unit 20may then undock from inventory holder 30, as described below, or moveinventory holder to another destination. For example, mobile drive unit20 may move inventory holder 30 to a packing station where a packer canselect appropriate inventory items 40 from inventory holder 30. Mobiledrive unit 20 may then return inventory holder 30 to its originallocation or another location appropriate for undocking, such as a newstorage location reserved for inventory holder 30.

FIG. 11 illustrates mobile drive unit 20 and inventory holder 30 whenthe two components arrive at an appropriate point for undocking. Asnoted above, this may represent a final destination specified by theoriginal command, the original storage location for inventory holder 30,or any other point within the workspace. At or near the destination,mobile drive unit 20 may detect another fiducial mark 450, fiducial mark450B, associated with the undocking location. Mobile drive unit 20determines its location based on fiducial mark 450B and, as a result,determines that it has reached the undocking location. After determiningthat it has reached the undocking location, mobile drive unit 20initiates an appropriate undocking process based on the configurationand characteristics of mobile drive unit 20 and inventory holder 30.

FIG. 12 illustrates a mobile drive unit 20 and inventory holder 30subsequent to undocking. Mobile drive unit 20 may then move away, assuggested by arrow 430, from inventory holder 30 and begin responding toother commands received by mobile drive unit 20.

As described above, embodiments herein are directed to detecting loadcharacteristics for inventory holders 30 and/or techniques formitigating negative effects which may arise from uneven or unbalancedload distribution in inventory holders 30.

FIG. 13 illustrates an example arrangement of load units 502 a-d(collectively, load units 502) and corresponding features 514 a-d(collectively, corresponding features 514) of an inventory holder 530for stabilization and/or detection of load distribution. The mobiledrive unit 520 and the inventory holder 530 can include features orelements of like names described elsewhere herein. For example, themobile drive unit 520 can include a holder sensor 548 capable ofdetecting holder identifier 560 to determine a position of the mobiledrive unit 520 relative to the inventory holder 530.

The docking head 510 of the mobile drive unit 520 can include any numberof load units 502 for determining load distribution of the inventoryholder 530. The load units 502 may interact with the docking surface 550and/or corresponding features 514 in order to detect load distribution.Load units 502 may alternatively or additionally provide a mechanism forsecuring the docking surface 550 of the inventory holder 530 to thedocking head 510 of the mobile drive unit 520, such as to improve adocking connection for heavy or unbalanced loads.

In some embodiments, the load units 502 include magnets, and thecorresponding features 514 include ferromagnetic material and/ormagnets. In a first example, the load units 502 are permanent magnetsthat engage with ferromagnetic material or magnets of the correspondingfeatures 514 to magnetically couple the docking head 510 with thedocking surface 550. The magnets may be of sufficient strength tomagnetically couple with corresponding features 514 of the inventoryholder 530, yet of low-enough strength to allow the weight of the mobiledrive unit 520 to provide a sufficient opposite force to result inseparation of the mobile drive unit 520 from the inventory holder 530when decoupling is desired. As an illustrative example, a set of magnetsfor a mobile drive unit 520 weighing 450 pounds may be chosen orcalibrated so as to have a maximum net magnetic coupling force of 450pounds or less. If a coupled inventory holder 530 began to tip over,such an arrangement would allow the magnetic connection to break ratherthan causing the mobile drive unit 520 to tip over as well.

The corresponding features 514 may be discrete features making up lessthan the entire docking surface 550, or alternatively, may cover or makeup the entire docking surface 550. Magnetically coupling the dockinghead 510 with the docking surface 550 can improve a stability of theinventory holder 530 when being lifted or moved by the mobile drive unit520. For example, magnetic coupling between the inventory holder 530 andthe mobile drive unit 520 may provide sufficient stability for themobile drive unit 520 to pick up and move an unbalanced inventory holder530 having a center of mass that is not located approximately at thegeometrical center of the inventory holder 530. Magnetic coupling mayalso permit the mobile drive unit 520 to transport the inventory holder530 at greater speeds than would otherwise be possible without magneticcoupling.

In another example, the load units 502 can include electromagnets. Theelectromagnets may be activated in order to engage magneticcorresponding features 514 of the docking surface 550. Alternatively,the load units 502 can include permanent magnets for docking with themagnetic corresponding features 514 of the docking surface 550 and alsoinclude electromagnets for providing a contrary, opposed magnetic fieldto counteract the permanent magnets for disengaging the docking head 510from the docking surface 550. Such an arrangement may reduce an energyconsumption of the electromagnets as a result of the electromagnets onlyoperating at the time undocking is performed, rather than the entiretime that the docking surface 550 and the docking head 510 are engaged.Any suitable combination of permanent magnets, electromagnets, and/orferromagnetic material can be included in the load units 502 and/orcorresponding features 514 to provide desired docking and/or undockingfunctionality. For example, in one embodiment, a load unit 502 aincludes a permanent magnet and/or electromagnet and the correspondingfeature 514 a includes ferromagnetic material, while another embodimentmay additionally or alternatively feature a reversed arrangement, inwhich ferromagnetic material is instead included in a load unit 502 band a permanent magnet and/or electromagnet is instead included in thecorresponding feature 514 b. As a further example, in some embodiments,an electromagnet included in either a load unit 502 c or a correspondingfeature 514 c may be powered by a power source included in the other ofthe load unit 502 c or the corresponding feature 514 c.

In addition to, or as an alternative to, permanent magnets,electromagnets, and/or ferromagnetic material for securing the mobiledrive unit 520 to the inventory unit 530, load units 502 may includepermanent magnets, electromagnets, and/or ferromagnetic material fordetecting a load distribution of weight in the inventory holder 530. Anindividual magnetic load unit 502 may be configured to detectdifferences in magnetic fields that correspond to differences in loaddistribution at the position of the magnetic load unit 502. In oneexample, magnets of the load unit 502 may be movable, such as a resultof being coupled with a calibrated spring that can indicate an amountthat the magnet moves. Alternatively, Hall Effect sensors may be used todetermine changes in magnetic fields resulting from magnets displacingunder load. As another alternative, magnets in the load units 502 may beplaced so as to trigger reed switches (e.g., switches that can betripped by a magnetic field) to indicate that a portion of the dockingsurface 550 is exerting sufficient pressure on the docking head 510 todisplace a magnet to trip the reed switch.

In some embodiments, load units 502 may include a camera or otherimaging device for determining a load distribution or center of mass ofthe inventory holder 530. The camera of a load unit 502 may be, but neednot necessarily be, the same camera as used in a holder position sensor548 (if such an arrangement is used). A load unit 502 may alternativelyinclude an additional camera or imaging sensor beyond that used in theinventory holder position sensor 548. The camera of a load unit 502 maycapture a series of images showing the position of a reference item(such as a corresponding feature 514 or marker 560) during a timeinterval. Changes in the position of the reference item in the imagesmay indicate corresponding changes in the position of the inventoryholder 530 relative to the mobile drive unit 520 during the timeinterval. The changing relative position of the inventory holder 530 mayprovide a measurement of the stability of the inventory holder 530. Forexample, the changing relative position of the inventory holder 530 canbe compared to acceleration data of the mobile drive unit 520 in thetime interval to determine how the inventory holder 530 is responding tothe acceleration. In some aspects, the response of the inventory holder530 to known acceleration may be used to determine a load distributionor center of mass of the inventory holder 530.

In some embodiments, the load units 502 include pressure sensors fordetermining load distribution of the inventory holder 530. For example,pressure sensors may be distributed across a face of the docking head510 of the mobile drive unit 520. Non-limiting examples of pressuresensors that may be utilized include calibrated monitored springs,piezoresistive, capacitive, electromagnetic, piezoelectric, optical,potentiometric, resonant, thermal, and ionization-based sensors.Differences in pressure detected by pressure sensors distributed acrossthe docking head 510 may indicate a load distribution of the inventoryholder 530 across the docking head 510. For example, a load unit 502 aregistering a greater pressure than a load unit 502 c may indicate thatthe center of mass of the inventory holder 530 is biased toward the loadunit 502 a, e.g., the right side of mobile drive unit 520 of FIG. 13. Aload unit 502 b registering a greater pressure than a load unit 502 dmay indicate that the center of mass of the inventory holder 530 isbiased toward the load unit 502 b, e.g., the near side of mobile driveunit 520 of FIG. 13. In this way, a position of the center of mass in anapproximately horizontal plane of the docking head 510 may bedetermined. Differences in pressure detected by load units over time mayindicate a response of the mass of the inventory holder 530 toacceleration, which may be utilized to determine a vertical position ofthe center of mass relative to the height of the inventory holder 530.

In some embodiments, the load units 502 include electrical sensors andthe corresponding features 514 include electrical contacts. Theelectrical sensors may detect differences in electrical connectivitybetween the mobile drive unit 520 and a docking surface 550 of theinventory holder 530 resulting from differences in load distribution.Electrical sensors of the load units 502 may provide an electricalcharge that may be received and/or transmitted by the electricalcontacts of the corresponding features 514. Based on the loaddistribution of the inventory holder 530, the electrical contacts of thecorresponding features 514 may displace into or out of contact with theelectrical sensors of the load units 502, thereby connecting or breakingcircuits so as to provide an indication of the distribution of load orweight among various areas of the docking surface 550. The electricalcontacts of the corresponding features 514 that engage the electricalsensors of the load units 502 may be evaluated to determine adistribution of weight and/or a position of a center of mass of theinventory holder 530.

FIG. 14 illustrates components of a resource scheduling module 1492according to a particular embodiment. The resource scheduling module1492 may provide similar functions as the resource scheduling module 92described with respect to FIG. 3, and may further provide functionalityrelated to load distribution of inventory holders 530. The resourcescheduling module 1492 may include a sensing module 1402, a loaddistribution module 1404, an instructions module 1406, and aninstructions database 1408. The sensing module 1402 can interface withother components of the inventory system 10, such as load units 502and/or corresponding features 514, to obtain information from sensorsabout load distribution of inventory holders 530 relative to mobiledrive unit 520. The load distribution module 1404 can processinformation from the sensing module 1402, such as determining the centerof mass of inventory holders 530 relative to mobile drive unit 520 basedon the acquired sensor information. The instructions module 1406 canaccess instructions database 1408 and provide instructions to mobiledrive units 520, other components, and/or operators of the inventorysystem 10 based on information from the load distribution module 1404.

FIG. 15 is a flowchart illustrating an example process 1500 that can beperformed by the inventory system 10 according to a particularembodiment. At 1510, the sensing module 1402 can instruct the mobiledrive unit 520 to utilize components (such as some combination of theload units 502 and/or corresponding features 514 described with respectto FIG. 13) to obtain information about load distribution of theinventory holder 530 relative to the mobile drive unit 520 and/orreceive such information from the mobile drive unit 520. For example, asdescribed above, the sensing module 1402 may obtain information from anycombination of magnets, imaging devices, pressure sensors, electricalcontacts, or other features associated with load units 502 and/orcorresponding features 514.

At 1520, the load distribution module 1404 can determine a center ofmass or other load distribution metric for the inventory holder 530. Forexample, in some embodiments, the load units 502 and/or correspondingfeatures 514 can provide load distribution data that can be utilized todetermine a status of an inventory holder 530 and/or alignment relativeto a mobile drive unit 520. As one example, the load distribution module1404 may determine that the load distribution data indicates that themobile drive unit 520 is engaged with or connected to the inventoryholder 530. As another example, the load distribution data may indicatethat the inventory holder 530 is unbalanced relative to the mobile driveunit 520. As further example, the load distribution data may indicatethat the inventory holder 530 is tipping over (or has tipped over)relative to the mobile drive unit 520.

At 1530, the instructions module 1406 can provide instructions (such asobtained from the instructions database 1408) to the mobile drive unit520 based on the load distribution of the inventory holder. Although insome embodiments, the instructions may be provided based on a calculatedload distribution from a single moment in time, in some embodiments, theinstructions may be provided based on a dynamic or ongoing calculationof the load distribution, such as over an extended period of time.

A variety of different responses may be instructed based on a detectedload distribution of an inventory holder 530 and/or a determination thatthe inventory holder 530 is unbalanced. In some embodiments, theinstructions module 1406 may instruct a change in destination of aninventory holder 530 based on the load distribution. For example, theinstructions module 1406 may send instructions to the mobile drive unit520 to move the inventory holder 530 to a station 50 at which aparticular item(s) that is causing an imbalance in the inventory holder530 can be removed or adjusted, either automatically,semi-automatically, or manually.

In some embodiments, the instructions module 1406 may instruct themobile drive unit 520 to alter a manner of docking with the inventoryholder 530. As one non-limiting example, the mobile drive unit 520 maybe instructed to disengage the docking head 510 from the docking surface550 and to re-engage the inventory holder 530 at a position closer todirectly beneath the center of mass of the inventory holder 530. Asanother non-limiting example, the mobile drive unit 520 may beinstructed to use magnets to more securely engage the inventory holder530. As a further non-limiting example, the mobile drive unit 520 may beinstructed retain the inventory holder 530 in a lifted state until anunbalanced status is resolved. Such an arrangement may permit anoperator to reach the inventory holder 530 and resolve the unbalancedstatus and/or may permit the mobile drive unit 520 to utilize magnets orother connection-improving mechanisms to secure and move the inventoryholder 530 until the unbalanced status is resolved by addition and/orremoval of inventory items 40 during the normal operation of theinventory system 10.

In some embodiments, the instructions module 1406 may instruct themobile drive unit 520 to change a velocity of travel while transportingthe inventory holder 530 based on the mass distribution of the inventoryholder 530. As non-limiting examples, the mobile drive unit 520 may beinstructed to increase speed when the inventory holder 530 iswell-balanced and is unlikely to tip over at higher speeds, and/or todecrease speed when the inventory holder is less balanced and more proneto tipping over at higher speeds.

In some embodiments, the load distribution module 1404 may receiveinformation about the load distribution on the inventory holder 530 andcompare that data to data about the inventory items 40 that are storedin the inventory holder 530. For example, the total weight of aninventory holder 530 (including the weight of the inventory items 40stored therein) that is measured by the load units 502 can be comparedwith stored information about the particular contents of the inventoryholder 530 to determine if weight estimates stored for the particularcontents are accurate.

In some embodiments, the mobile drive unit 520 may also release anymagnetic coupling with the inventory holder 530 in response to adetermination that the inventory holder 530 is tipping over relative tothe mobile drive unit 520. This may be a useful action to prevent themobile drive unit 520 from also tipping over along with the inventoryholder 530.

In some embodiments, the instructions module 1406 may instruct themobile drive unit 520 to move in an abrupt manner that is likely toadjust and/or improve a load distribution of the inventory holder 530.For example, the mobile drive unit 520 may be instructed to turnsuddenly, accelerate, decelerate, set down or drop the inventory holder530 abruptly, spin, and/or perform other motions likely to shift theposition of target inventory items 40 stored on the inventory holder530.

In some embodiments, the mobile drive unit 520 can be instructed tointeract with or engage, or cause the inventory holder 530 to interactwith or engage, an obstacle. Non-limiting examples of obstacles includea bump (such as shown in FIG. 1), a bar positioned above a floor at aheight close to a shelf on which the inventory item is positioned, awall, another mobile drive unit 520, and another inventory holder 530.As an illustrative example, in some aspects, an inventory holder 530 maybe collided with another inventory holder 530 in order to transferinventory items 40 or adjust and/or secure inventory items 40 in one orthe other of the inventory holders 530 that are collided together.

In some embodiments, the mobile drive unit 520 may be instructed to takethe inventory holder 530 to a location for subsequent actions. Forexample, the mobile drive unit 520 may take the inventory holder 530 toa station where an operator may improve a load distribution or balanceof the inventory holder 530 by adjusting the distribution of inventoryitems 40 stored in the inventory holder 530. As another example, aninventory holder 530 may be carried by a mobile drive unit 520 to avibration plate or other mechanism configured to jostle the inventoryholder 530, with or without the mobile drive unit 520, and without usingthe mobile drive unit 520 to impart the jostling motion to the inventoryholder 530. In some embodiments, the station, a mobile drive unit, aninventory holder, and/or other components particularly configured toaccommodate collisions, jostling, or vibrating inventory items 40relative to the inventory holder 530 can include padding or nettingarranged to receive inventory items 40 falling and/or moving as a resultof the collisions, jostling, or vibrating. In some embodiments,collisions, jostling, or vibrating may be performed in order topreemptively cause any loose or unbalanced inventory items 40 to becomedisplaced from the inventory holder 530. Such preemptive action maycause displacement of inventory items 40 at a location where thedisplacement is easily handled (such as at a station 50, where anoperator can recover and/or rearrange the problematic inventory items40), rather than at a more problematic location (such as remote from astation 50, where a displacement may interfere with normal operation ofan inventory system 10 and be difficult for an operator to reach).

In some embodiments, based on inventory data about inventory items 40stored in an inventory holder 530, the management module 15 may analyzea risk of performing such collisions, jostling, or vibrating. Forexample, the management module 15 may determine a risk of damage basedon whether the inventory items 40 are fragile, contain liquid or areotherwise at high risk. The management module 15 may determine that therisk of damaged inventory items 40 outweighs the benefit of rebalancinginventory items 40 on an inventory holder 530 and instruct the mobiledrive unit 520 not to perform such an action based on thatdetermination.

FIG. 16 illustrates aspects of an example environment 1600 forimplementing aspects in accordance with various embodiments. As will beappreciated, although a Web-based environment is used for purposes ofexplanation, different environments may be used, as appropriate, toimplement various embodiments. The environment includes an electronicclient device 1602, which can include any appropriate device operable tosend and receive requests, messages, or information over an appropriatenetwork 1604 and convey information back to a user of the device.Examples of such client devices include personal computers, cell phones,handheld messaging devices, laptop computers, set-top boxes, personaldata assistants, electronic book readers, and the like. The network caninclude any appropriate network, including an intranet, the Internet, acellular network, a local area network or any other such network orcombination thereof. Components used for such a system can depend atleast in part upon the type of network and/or environment selected.Protocols and components for communicating via such a network are wellknown and will not be discussed herein in detail. Communication over thenetwork can be enabled by wired or wireless connections and combinationsthereof. In this example, the network includes the Internet, as theenvironment includes a Web server 1606 for receiving requests andserving content in response thereto, although for other networks analternative device serving a similar purpose could be used as would beapparent to one of ordinary skill in the art.

The illustrative environment includes at least one application server1608 and a data store 1610. It should be understood that there can beseveral application servers, layers, or other elements, processes orcomponents, which may be chained or otherwise configured, which caninteract to perform tasks such as obtaining data from an appropriatedata store. As used herein the term “data store” refers to any device orcombination of devices capable of storing, accessing, and retrievingdata, which may include any combination and number of data servers,databases, data storage devices and data storage media, in any standard,distributed or clustered environment. The application server can includeany appropriate hardware and software for integrating with the datastore as needed to execute aspects of one or more applications for theclient device, handling a majority of the data access and business logicfor an application. The application server provides access controlservices in cooperation with the data store and is able to generatecontent such as text, graphics, audio and/or video to be transferred tothe user, which may be served to the user by the Web server in the formof HyperText Markup Language (“HTML”), Extensible Markup Language(“XML”) or another appropriate structured language in this example. Thehandling of all requests and responses, as well as the delivery ofcontent between the client device 1602 and the application server 1608,can be handled by the Web server. It should be understood that the Weband application servers are not required and are merely examplecomponents, as structured code discussed herein can be executed on anyappropriate device or host machine as discussed elsewhere herein.

The data store 1610 can include several separate data tables, databasesor other data storage mechanisms and media for storing data relating toa particular aspect. For example, the data store illustrated includesmechanisms for storing information which can be used by modulesdescribed herein, such as resource scheduling information 1612, routeplanning information 1614, segment reservation information 1616, and/orinventory information 1618. It should be understood that there can bemany other aspects that may need to be stored in the data store, such asfor page image information and to access right information, which can bestored in any of the above listed mechanisms as appropriate or inadditional mechanisms in the data store 1610. The data store 1610 isoperable, through logic associated therewith, to receive instructionsfrom the application server 1608 and obtain, update or otherwise processdata in response thereto.

Each server typically will include an operating system that providesexecutable program instructions for the general administration andoperation of that server and typically will include a computer-readablestorage medium (e.g., a hard disk, random access memory, read onlymemory, etc.) storing instructions that, when executed by a processor ofthe server, allow the server to perform its intended functions. Suitableimplementations for the operating system and general functionality ofthe servers are known or commercially available and are readilyimplemented by persons having ordinary skill in the art, particularly inlight of the disclosure herein.

The environment in one embodiment is a distributed computing environmentutilizing several computer systems and components that areinterconnected via communication links, using one or more computernetworks or direct connections. However, it will be appreciated by thoseof ordinary skill in the art that such a system could operate equallywell in a system having fewer or a greater number of components than areillustrated in FIG. 16. Thus, the depiction of the system 1600 in FIG.16 should be taken as being illustrative in nature and not limiting tothe scope of the disclosure.

The various embodiments further can be implemented in a wide variety ofoperating environments, which in some cases can include one or more usercomputers, computing devices or processing devices which can be used tooperate any of a number of applications. User or client devices caninclude any of a number of general purpose personal computers, such asdesktop or laptop computers running a standard operating system, as wellas cellular, wireless and handheld devices running mobile software andcapable of supporting a number of networking and messaging protocols.Such a system also can include a number of workstations running any of avariety of commercially-available operating systems and other knownapplications for purposes such as development and database management.These devices also can include other electronic devices, such as dummyterminals, thin-clients, gaming systems and other devices capable ofcommunicating via a network.

Most embodiments utilize at least one network that would be familiar tothose skilled in the art for supporting communications using any of avariety of commercially-available protocols, such as TransmissionControl Protocol/Internet Protocol (“TCP/IP”), Open SystemInterconnection (“OSI”), File Transfer Protocol (“FTP”), Universal Plugand Play (“UpnP”), Network File System (“NFS”), Common Internet FileSystem (“CIFS”) and AppleTalk. The network can be, for example, a localarea network, a wide-area network, a virtual private network, theInternet, an intranet, an extranet, a public switched telephone network,an infrared network, a wireless network, and/or any combination thereof.

In embodiments utilizing a Web server, the Web server can run any of avariety of server or mid-tier applications, including Hypertext TransferProtocol (“HTTP”) servers, FTP servers, Common Gateway Interface (“CGI”)servers, data servers, Java servers and business application servers.The server(s) also may be capable of executing programs or scripts inresponse requests from user devices, such as by executing one or moreWeb applications that may be implemented as one or more scripts orprograms written in any programming language, such as Java®, C, C# orC++, or any scripting language, such as Perl, Python or TCL, as well ascombinations thereof. The server(s) may also include database servers,including without limitation those commercially available from Oracle®,Microsoft®, Sybase®° and IBM®.

The environment can include a variety of data stores and other memoryand storage media as discussed above. These can reside in a variety oflocations, such as on a storage medium local to (and/or resident in) oneor more of the computers or remote from any or all of the computersacross the network. In a particular set of embodiments, the informationmay reside in a storage-area network (“SAN”) familiar to those skilledin the art. Similarly, any necessary files for performing the functionsattributed to the computers, servers or other network devices may bestored locally and/or remotely, as appropriate. Where a system includescomputerized devices, each such device can include hardware elementsthat may be electrically coupled via a bus, the elements including, forexample, at least one central processing unit (“CPU”), at least oneinput device (e.g., a mouse, keyboard, controller, touch screen orkeypad) and at least one output device (e.g., a display device, printeror speaker). Such a system may also include one or more storage devices,such as disk drives, optical storage devices and solid-state storagedevices such as random access memory (“RAM”) or read-only memory(“ROM”), as well as removable media devices, memory cards, flash cards,etc.

Such devices also can include a computer-readable storage media reader,a communications device (e.g., a modem, a network card (wireless orwired), an infrared communication device, etc.) and working memory asdescribed above. The computer-readable storage media reader can beconnected with, or configured to receive, a computer-readable storagemedium, representing remote, local, fixed, and/or removable storagedevices as well as storage media for temporarily and/or more permanentlycontaining, storing, transmitting, and retrieving computer-readableinformation. The system and various devices also typically will includea number of software applications, modules, services or other elementslocated within at least one working memory device, including anoperating system and application programs, such as a client applicationor Web browser. It should be appreciated that alternate embodiments mayhave numerous variations from that described above. For example,customized hardware might also be used and/or particular elements mightbe implemented in hardware, software (including portable software, suchas applets) or both. Further, connection to other computing devices suchas network input/output devices may be employed.

Storage media and computer readable media for containing code, orportions of code, can include any appropriate media known or used in theart, including storage media and communication media, such as but notlimited to volatile and non-volatile, removable and non-removable mediaimplemented in any method or technology for storage and/or transmissionof information such as computer readable instructions, data structures,program modules or other data, including RAM, ROM, Electrically ErasableProgrammable Read-Only Memory (“EEPROM”), flash memory or other memorytechnology, Compact Disc Read-Only Memory (“CD-ROM”), digital versatiledisk (DVD) or other optical storage, magnetic cassettes, magnetic tape,magnetic disk storage or other magnetic storage devices or any othermedium which can be used to store the desired information and which canbe accessed by the a system device. Based at least in part on thedisclosure and teachings provided herein, a person of ordinary skill inthe art will appreciate other ways and/or methods to implement thevarious embodiments.

The specification and drawings are, accordingly, to be regarded in anillustrative rather than a restrictive sense. It will, however, beevident that various modifications and changes may be made thereuntowithout departing from the broader spirit and scope of the disclosure asset forth in the claims.

Other variations are within the spirit of the present disclosure. Thus,while the disclosed techniques are susceptible to various modificationsand alternative constructions, certain illustrated embodiments thereofare shown in the drawings and have been described above in detail. Itshould be understood, however, that there is no intention to limit theinvention to the specific form or forms disclosed, but on the contrary,the intention is to cover all modifications, alternative constructionsand equivalents falling within the spirit and scope of the invention, asdefined in the appended claims.

The use of the terms “a” and “an” and “the” and similar referents in thecontext of describing the disclosed embodiments (especially in thecontext of the following claims) are to be construed to cover both thesingular and the plural, unless otherwise indicated herein or clearlycontradicted by context. The terms “comprising,” “having,” “including,”and “containing” are to be construed as open-ended terms (i.e., meaning“including, but not limited to,”) unless otherwise noted. The term“connected” is to be construed as partly or wholly contained within,attached to, or joined together, even if there is something intervening.Recitation of ranges of values herein are merely intended to serve as ashorthand method of referring individually to each separate valuefalling within the range, unless otherwise indicated herein and eachseparate value is incorporated into the specification as if it wereindividually recited herein. All methods described herein can beperformed in any suitable order unless otherwise indicated herein orotherwise clearly contradicted by context. The use of any and allexamples, or exemplary language (e.g., “such as”) provided herein, isintended merely to better illuminate embodiments of the invention anddoes not pose a limitation on the scope of the invention unlessotherwise claimed. No language in the specification should be construedas indicating any non-claimed element as essential to the practice ofthe invention.

Preferred embodiments of this disclosure are described herein, includingthe best mode known to the inventors for carrying out the invention.Variations of those preferred embodiments may become apparent to thoseof ordinary skill in the art upon reading the foregoing description. Theinventors expect skilled artisans to employ such variations asappropriate and the inventors intend for the invention to be practicedotherwise than as specifically described herein. Accordingly, thisinvention includes all modifications and equivalents of the subjectmatter recited in the claims appended hereto as permitted by applicablelaw. Moreover, any combination of the above-described elements in allpossible variations thereof is encompassed by the invention unlessotherwise indicated herein or otherwise clearly contradicted by context.

All references, including publications, patent applications, andpatents, cited herein are hereby incorporated by reference to the sameextent as if each reference were individually and specifically indicatedto be incorporated by reference and were set forth in its entiretyherein.

What is claimed is:
 1. A mobile drive unit comprising: a drive modulecomprising one or more wheels configured to propel the mobile driveunit; and a docking head configured to engage a frame of an inventoryholder for lifting the inventory holder, the inventory holder having acenter of mass dependent on a distribution of any inventory items storedin the inventory holder, the docking head comprising one or more magnetsarranged so as to engage with a ferromagnetic surface of the frame ofthe inventory holder when the mobile drive unit is lifting the inventoryholder, the one or more magnets configured to (i) secure the inventoryholder to the mobile drive unit during lifting and/or moving by themobile drive unit, and (ii) provide information about a position of thecenter of mass of the inventory holder relative to the mobile drive unitwhen lifted by the mobile drive unit.
 2. The mobile drive unit of claim1, wherein the docking head comprises: a first set of magnets configuredto secure the inventory holder to the mobile drive unit during liftingand/or moving by the mobile drive unit; and a second set of magnetsconfigured to provide information about the position of the center ofmass of the inventory holder relative to mobile drive unit when liftedby the mobile drive unit.
 3. The mobile drive unit of claim 1, whereinthe mobile drive unit further comprises a module configured to at leastone of receive or generate a determination that the inventory holder isconnected to the mobile drive unit, the determination based on theprovided information about the position of the center of mass of theinventory holder relative to mobile drive unit.
 4. The mobile drive unitof claim 1, wherein the mobile drive unit further comprises a moduleconfigured to at least one of receive or generate a determination thatthe inventory holder is unbalanced relative to the mobile drive unit,the determination based on the provided information about the positionof the center of mass of the inventory holder relative to mobile driveunit.
 5. The mobile drive unit of claim 1, wherein the mobile drive unitfurther comprises a module configured to at least one of receive orgenerate a determination that the inventory holder is tipping over orhas tipped over relative to the mobile drive unit, the determinationbased on the provided information about the position of the center ofmass of the inventory holder relative to mobile drive unit.
 6. A mobiledrive unit comprising: a drive module comprising a propulsion systemconfigured to move the mobile drive unit between locations of aworkspace; a docking head configured to dock with an inventory holderfor movement of the inventory holder by the mobile drive unit, theinventory holder having a center of mass dependent on a distribution ofany inventory items stored in the inventory holder; a sensor configuredto obtain information about a load distribution of the inventory holderrelative to the mobile drive unit, and a module configured to at leastone of receive or generate instructions based on the informationobtained by the sensor.
 7. The mobile drive unit of claim 6, wherein thesensor comprises an imaging device configured to detect a change inposition of a reference item associated with a surface of the inventoryholder, the change resulting from the load distribution of the inventoryholder relative to the mobile drive unit.
 8. The mobile drive unit ofclaim 6, wherein the sensor comprises electrical sensors configured todetect differences in electrical connectivity between the mobile driveunit and a surface of the inventory holder resulting from the loaddistribution of the inventory holder relative to the mobile drive unit.9. The mobile drive unit of claim 6, wherein the sensor comprisespressure sensors configured to detect differences in pressure betweenthe mobile drive unit and a surface of the inventory holder resultingfrom the load distribution of the inventory holder relative to themobile drive unit.
 10. The mobile drive unit of claim 6, wherein thesensor comprises magnets configured to detect differences in magneticfields between the mobile drive unit and a surface of the inventoryholder resulting from the load distribution of the inventory holderrelative to the mobile drive unit.
 11. The mobile drive unit of claim 6,wherein the instructions instruct the mobile drive unit to disengage theinventory holder and re-engage the inventory holder at a differentposition for docking based on the information obtained by the sensor.12. The mobile drive unit of claim 6, wherein the instructions instructthe mobile drive unit to retain the inventory holder in a docked statebased on the information obtained by the sensor.
 13. The mobile driveunit of claim 6, wherein the instructions instruct the mobile drive unitto move abruptly so as to move at least one inventory item stored by theinventory holder.
 14. The mobile drive unit of claim 6, wherein theinstructions instruct the mobile drive unit to vary a speed of travel ofthe mobile drive unit based on the information obtained by the sensor.15. The mobile drive unit of claim 6, wherein the instructions instructthe mobile drive unit to alter a destination based on the informationobtained by the sensor.
 16. The mobile drive unit of claim 6, whereinthe module is further configured to at least one of receive or generateinformation for: determining measured load distribution informationabout the inventory holder and inventory items stored therein based onthe information obtained by the sensor; and evaluating estimated weightinformation about an estimated weight of the inventory items stored inthe inventory holder based on the measured load distributioninformation.
 17. A mobile drive unit comprising: a propulsion systemconfigured to move the mobile drive unit between locations of aworkspace; a docking assembly configured to couple with a frame of aninventory holder for movement of the inventory holder between locationsof the workspace by the propulsion system of the mobile drive unit; anda first magnetic element connected to the docking assembly andcomprising one of a ferromagnetic surface or one or more magnets, thefirst magnetic element configured to magnetically couple with a secondmagnetic element that (i) is connected to the frame of the inventoryholder and (ii) comprises another of the ferromagnetic surface or theone or more magnets, the first magnetic element configured tomagnetically couple with the second magnetic element so as to improve astrength with which the mobile drive unit couples with the inventoryholder for movement of the inventory holder.
 18. The mobile drive unitof claim 17, wherein said one or more magnets comprise one or morepermanent magnets having a combined magnetic force less than a weight ofthe mobile drive unit.
 19. The mobile drive unit of claim 17, whereinsaid one or more magnets comprise one or more electromagnets configuredto, when activated, magnetically couple the inventory holder to themobile drive unit.
 20. The mobile drive unit of claim 17, wherein saidone or more magnets comprise one or more permanent magnets configured tomagnetically couple the mobile drive unit with the inventory holder andone or more electromagnets configured to, when activated, magneticallydecouple the mobile drive unit from the inventory holder.